1. Field of the Invention
This invention relates to a positive temperature coefficient (FTC) circuit protection device, more particularly to a FTC circuit protection device including a polymer matrix formed from a co-melted mixture of a base polyolefin and a reinforcing polyolefin.
2. Description of the Related Art
A positive temperature coefficient (PTC) element exhibits a PTC effect that renders the same to be useful as a circuit protecting device, such as a resettable fuse. The PTC element includes a PTC polymer material and first and second electrodes attached to two opposite surfaces of the PTC polymer material. The PTC polymer material includes a polymer matrix that contains a crystalline region and a non-crystalline region, and a particulate conductive filler dispersed in the non-crystalline region of the polymer matrix and formed into a continuous conductive path for electrical conduction between the first and second electrodes. The PTC effect is a phenomena that when the temperature of the polymer matrix is raised to its melting point, crystals in the crystalline region start melting, which results in generation of a new non-crystalline region. As the new non-crystalline region is increased to an extent to merge into the original non-crystalline region, the conductive path of the particulate conductive filler will become discontinuous and the resistance of the PTC polymer material will sharply increase, thereby resulting in an electrical disconnection between the first and second electrodes.
Conventionally, the polymer matrix is made from a polymer composition containing a base high density polyethylene (HDPE) having a weight average molecular weight ranging from 50,000 g/mole to 300,000 g/mole and a melt flow rate ranging from 0.01 g/10 min to 10 g/10 min according to ASTM D-1238 under 190° C. and a load of 2.16 Kg, and optionally a carboxylic acid anhydride grafted HDPE having a weight average molecular weight ranging from 50,000 g/mole to 200,000 g/mole and a melt flow rate ranging from 0.5 g/10 min to 10 g/10 min according to ASTM D-1238 under 190° C. and a load of 2.16 Kg. The grafted HDPE serves to increase adhesion of the PTC polymer material to the electrodes.
Examples of the particulate conductive filler are carbon black, metal powders, conductive ceramic powders, metalized glass beads, etc. Since carbon black has a lower conductivity, the PTC polymer materials using carbon black as the particulate conductive filler will have a resistivity greater than 0.1 ohm-cm at room temperature. Hence, for PTC circuit protection devices that require the PTC polymer materials to have a resistivity less than 0.1 ohm-cm or even less 0.05 ohm-cm, carbon black is no longer suitable for use as the particulate conductive filler. Although the conductivity of the PTC polymer material can be considerably increased by using the non-carbon particulate conductive fillers, such as metal powders, these highly conductive non-carbon particulate conductive fillers tend to result in undesired generation of electric arc within the PTC polymer material during use. The electric arc thus formed can deteriorate the molecular structure of the polymermatrix of the PTC polymer material, which can result in an unstable electrical property of the PTC element and a decrease in the service life of the PTC element. Hence, there is a need to improve the conductivity of the PTC polymer material without resulting in deterioration of the PTC polymer material.